The instant invention relates to the field of switching power supply, for example of the type usually used in TV sets.
FIG. 1 is intended to remind some of the main characteristics of a switching power supply. The latter mainly comprises a transformer T, the primary winding T.sub.1 of which is connected in series to a power switch, usually a power transistor 1. Thus, current flows in the primary winding T.sub.1 between a power supply terminal 2 and the ground when the transistor 1 is switched on. In operation, transistor 1 receives repetitive base control pulses from an integrated circuit 3 and therefore ensures during each of its conduction phases energy transmission from the primary winding T.sub.1 to the secondary windings T.sub.2 of transformer T. Reference T.sub.3 designates a specific secondary winding which is connected through a diode 5 to the integrated circuit of the base control 3 for ensuring its power supply. Thus, the integrated circuit 3 is fed only once the switching power supply is operating. At the switching on of the system, this integrated circuit is not fed by the secondary winding T3. Therefore, one generally provides for an auxiliary circuit comprising a capacitor 7 charged through a resistor 9 connected to the power supply terminal 2 in order to ensure the initial current supply of the circuit before the start up of the switching power supply.
The integrated circuit 3 receives on a terminal 10 servo control signals generated from the signal of at least one of the secondary windings T.sub.2, usually through another integrated circuit for supplying a control pulse sequence to the power transistor 1 in order to obtain a stabilized voltage at the secondary. The case under consideration relates to a pulse width modulation control (PWM).
FIG. 2A shows a portion of the integrated circuit 3 permitting the PWM. This figure essentially shows a comparator C comprising two transistors 11 and 12, the emitters of which are interconnected and receive the current from a current source 13 the collector of transistor 12 is directly grounded and the collector of transistor 11 is grounded through a resistor 14. This comparator comprises a first comparison input on the base of transistor 11 corresponding to the terminal 10 hereinabove mentioned, a second comparison input 15 on the base of transistor 12 receiving a saw-tooth signal, and an output 16 on the collector of transistor 11.
FIG. 2B schematically shows the incoming saw-tooth signal on terminal 15 and the servo control voltage on terminal 10. FIG. 2C shows the output pulses on terminal 16 designed to control (through amplifying circuits) the base of the power transistor 1. In the specific circuit illustrated here, the nearest the servo control voltage 10 is to the low threshold of the saw-tooth 15, the largest the width of pulses 16 is. Of course, this is an arbitrary choice and an inverse system is liable to be provided for. However, this system will be chosen for the description of the invention that will follow.
At the start up of the switching power supply, the secondary windings are not fed and the servo control signal 10 will be set to a low level causing a maximum energy requirement. The conduction duration of transistor 1 will then be immediately maximal, whereby various drawbacks may arise correlated with the inductive components of the circuit and with saturation risks of the transformer.
In order to avoid those drawbacks, progressive start up circuits for the switching power supply have been provided for in the prior art, a schematical example of which is illustrated in FIG. 3A. FIG. 3A again shows the comparator C of FIG. 2A. However, the servo control signal 10, instead of being directly connected to the base of transistor 11, is connected to this base through an analog OR circuit 20, the function of which is to allow the higher of the two signals it receives on its inputs to flow. The second input 19 of the analog OR circuit 20 is connected to the connection point of a capacitor 21, the other terminal of which is connected to the high voltage source, and of a resistor 22, the other terminal of which is grounded. FIG. 3A also symbolically shows a voltage source 23 and a switch 24 which is switched ON when the circuit is started.
FIG. 3B shows the shape of the input signals on terminals 10, 15 and 19 and FIG. 3C shows the shape of the control pulses on the output terminal 16. At the switching ON time t.sub.0 of switch 24, terminal 19 is at a high voltage and this voltage drops while the capacitor 21 is being charged. The saw-tooth 15 starts appearing after a given time and, at a time t.sub.1, the signal on terminal 19 reaches the highest level of the saw-tooth 15. At that time, a first pulse is supplied at the output, that is, the switching power supply starts operating. Then, while the voltage on terminal 19 decreases, pulses having progressively increasing widths are supplied at the output terminal 16 until a time t.sub.2 when the servo control voltage on terminal 10 gets operative.
It will be noted that FIGS. 3B and 3C are shown for the sake of illustration and are not drawn to scale. Indeed, conventionally, in a TV switching power supply, the period of the saw-teeth corresponds to the TV line scanning period, that is, for example, 64 microseconds. The duration of the initial dead time between the times t.sub.0 and t.sub.1 is about hundred microseconds, and the duration of the progressive start up between the times t.sub.1 and t.sub.2 is also about hundred microseconds, that is, the decrease of signal 19 is spread over a few thousands of saw-tooth periods.
The various devices hereinabove described are commercially available and are for example used in the switching power supply circuits for TV sets marketed under references TEA 5170, UAA 4001, UAA 4006 by the company SGS-Thomson Microelectronics SA.
This circuit has proved satisfactory as regards the progressive start up between the times t.sub.1 and t.sub.2. However, it presents a drawback since there is a significant dead time between the times t.sub.0 and t.sub.1, that is, between the feed instant and the occurrence of the first control pulses.
In order to palliate this drawback, it has been proposed in the prior art to ensure an initial charge of the capacitor 21 close to the high threshold of the sawtooth with a circuit of the type illustrated in FIG. 4. This circuit again includes, in addition to the components already described in FIG. 3A, two resistors 31 and 32 connected in series between the high and low power supply terminals, the junction of which is connected to the junction of capacitor 21 and resistor 22 through a diode 33. Thus, the voltage on terminal 19, instead of starting from the high value V.sub.CC of the power supply voltage, starts from a value equal to V.sub.CC [R1/(R1+R2)]+VD, VD being the forward voltage drop in the diode 33. As a result, the period between the times t.sub.0 and t.sub.1 is decreased. Such a device is used for example in the circuits referenced TEA 2164 and TEA 2029 marketed by the company SGS-Thomson Microelectronics SA, the components 31, 32 and 33 being external to the integrated circuit for the TEA 2029 and internal for the TEA 2164. With this device, the dead time between the times t.sub.0 and t.sub.1 is reduced but the accuracy of the system is not very satisfactory since it depends upon the accuracy on the resistors R1 and R2 and upon that on the saw-tooth amplitude as well as upon the drift caused by the diode 33. For this reason, a security range is to be provided and there still remains a dead time, which is of course lowered with respect to that of the previous process, but which presents the drawback of being highly scattered from one circuit to another.
One of the drawbacks of the relatively high dead times is of course the annoyance caused to the televiewer who will have to wait for some time until he receives the picture signals. Another drawback, that can be seen in relation with the drawing of FIG. 1, is that, if a relatively high dead time is provided, the capacitor 7, ensuring the initial power supply of the integrated circuit as long as the switching power supply is not set to a steady state, will have to exhibit a very high value and the resistor 9 in series with this capacitor shall not have a too high value for permitting the rapid charge of capacitor 7 at the switching on. If the dead time is reduced, it will be possible to reduce the value of capacitor 7 and increase the value of resistor 9, which reduces the consumption during the steady state period.
Thus, an object of the instant invention is to reduce the duration of the dead time while ensuring a satisfactory progressive start up.
Another object of the instant invention is to provide for such a circuit, most components of which are liable to be realized in the form of integrated circuits.